Technological advances of all kinds continue to present many complex ecological issues. Consequently, waste management and pollution prevention are two very significant challenges of the 21st century. The overwhelming quantity of plastic refuse has significantly contributed to the critical shortage of landfill space faced by many communities. For example, poly(ethylene terephthalate (PET) (poly(oxy-1,2-ethanediyl-oxycarbonyl-1,4-diphenylenecarbony-1), a widely used engineering thermoplastic for carpeting, clothing, tire cords, soda bottles and other containers, film, automotive applications, electronics, displays, etc. will contribute more than 1 billion pounds of waste to land-fills in 2002 alone. The worldwide production of PET continues to grow at a high rate. Interestingly, the precursor monomers represent a small amount of the petrochemical stream. Moreover, the proliferation of the use of organic solvents, halogenated solvents, water, and energy consumption in addressing the need to recycle commodity polymers such as PET and other polyesters has created the need for environmentally responsible and energy efficient recycling processes. See Nadkarni (1999) International Fiber Journal 14(3).
Commercial synthesis of PET often involves a two-step transesterification process from dimethyl teraphthalate (DMT) and excess ethylene glycol (EO) in the presence of a metal alkanoates or acetates of calcium, zinc, manganese, titanium etc. This first step generates bis(hydroxy ethylene)teraphthalate (BHET) with the elimination of methanol and the excess EO. The BHET is heated, generally in the presence of a transesterification catalyst, to generate high polymer. This process is generally accomplished in a vented extruder to remove the polycondensate (EO) and generate the desired thermoformed object from a low viscosity precursor.
Effort has been invested in researching recycling strategies for PET, and these efforts have produced three commercial options; mechanical, chemical and energy recycling. Energy recycling simply burns the plastic for its calorific content. Mechanical recycling, the most widespread approach, often involves grinding the polymer to powder, which is then mixed with “virgin” PET. See Mancini et al., Materials Research 2(1):33-38 (1999). Many chemical companies use this process in order to recycle PET; however, it has been demonstrated that successive recycling steps cause significant polymer degradation, in turn resulting in a loss of desirable mechanical properties.
Recycling using chemical degradation involves a process that depolymerizes a polymer to starting material, or at least to relative short oligomeric components. Industrial processes for chemical recycling include glycolysis, methanolysis, and hydrolysis. It is known that methanolysis and glycolysis may be carried out using a catalyst comprising an N-heterocyclic carbene (NHC).

Chemical recycling processes are the most difficult to control, since in many cases elevated temperature and pressure are required along with a catalyst composed of a strong base, an organometallic complex, and/or a carbene compound. See Sako et al., Proc. of the 4th Int'l Symposium on Supercritical Fluids, pp. 107-110 (1997). In many instances, the use of such catalysts result in significant quantities of undesirable byproducts; such materials are generally unsuitable, for example, for use in medical materials or food packaging, limiting their utility. Also, where large amounts of energy are required to effect depolymerization, sustainability arguments for chemical depolymerization are essentially eliminated. Furthermore, some depolymerization catalysts are difficult to prepare, unstable to long-term storage, or require stringent reaction conditions; therefore, currently the chemical recycling of PET is not economically viable relative to mechanical recycling and is thus not widely practiced.
In addition, the low cost of starting monomer provides significant economic challenges for alternative technologies utilizing post-consumer PET as a feedstock. Incentives to develop better processes for chemical recycling remain high due to PET's increasing consumer use, the subsequent landfill disposal problem and PET's slow rate of natural degradation. Efforts in the chemical recycling of PET are thus ideally focused on developing an environmentally safe, economically feasible, and industrially applicable process for wide-scale application. Moreover, chemical recycling methodologies that are energy efficient and do not involve a heavy metals are highly desirable.